What kept Constantinople safe were its walls. Once the necessary cannon to knock down those walls come around, some power is going to take Constantinople eventually. I'd say it bought them a little time at most.
Why? How often did Comstantinople fall after the Turks took it? The city fell twice to foreign invaders (well, 1.5 times, since the Crusaders were originally putting a claimant on the throne), and only ome of those sieges involved gunpowder. After that, why, the city did just fine. Its only really at risk if its the center of a weakened state.